Conventionally, in a wire bonder (wire bonding device) that connects, using a wire, an electrode (pad) on an IC chip being a first bonding point and a lead being a second bonding point, discharge is firstly caused by applying high voltage between a leading end of a wire fed from a capillary and a discharge electrode, and then, the leading end of the wire is melted with discharge energy, so that a ball is formed at the leading end of the wire having passed through the capillary. Here, a ball formed at a leading end of a wire with discharge is called an initial ball.
FIG. 8 is a block diagram illustrating a structure of a conventional ball forming device for a wide bonder for forming a ball at a leading end of a capillary by applying high voltage between a leading end of a wire fed from the capillary and a discharge electrode.
As illustrated in FIG. 8, a conventional ball forming device 45 for a wire bonder includes a dielectric breakdown power source 54 to generate high voltage for breaking insulation between a wire 31 and a discharge electrode 33, a high voltage generating portion 50 to generate high voltage for causing discharge current to flow after dielectric breakdown, and a constant current switch 51 to control discharge current flowing between the wire 31 at a leading end of a capillary 32 and the discharge electrode 33.
The dielectric breakdown power source 54 is configured to generate high voltage for breaking insulation between the wire 31 and the discharge electrode 33 and be turned off after dielectric breakdown. The high voltage generating portion 50 is configured to be turned on concurrently with the dielectric breakdown power source 54 at the time of dielectric breakdown between the wire 31 and the discharge electrode 33 and generate high voltage after the dielectric breakdown as well.
A positive electrode terminal (+) of the high voltage generating portion 50 is configured to apply positive electrode voltage to the wire 31 having passed through the capillary 32 as a bonding tool from a clamper 30 through the constant current switch 51.
Meanwhile, a negative electrode terminal (−) of the high voltage generating portion 50 is configured to apply negative electrode voltage to the discharge electrode 33. A discharge stabilizing resistor 34 for stabilizing discharge is arranged between the negative electrode terminal (−) of the high voltage generating portion 50 and the discharge electrode 33. As illustrated in FIG. 8, cables are arranged to connect the clamper 30 with a terminal c and the discharge electrode 33 including the discharge stabilizing resistor 34 with a terminal d.
As illustrated in FIG. 8, a timer circuit 52 is connected to the constant current switch 51 for controlling switching of a switch circuit (not illustrated) built in the constant current switch 51. A trigger signal Tr being a discharge start signal is to be provided to the timer circuit 52.
The timer circuit 52 has a timer for setting a discharge duration and the timer is activated by the trigger signal Tr provided from the outside. The timer circuit 52 is caused to be turned on by the trigger signal Tr, so that the switch circuit of the constant current switch 51 is closed and current flows between the wire 31 and the discharge electrode 33 after dielectric breakdown. After a lapse of the predetermined discharge duration, the timer circuit 52 is caused to be turned off, so that the switch circuit is opened and current is blocked. The constant current switch 51 is configured to cause constant current to flow between the discharge electrode 33 and the wire 31 while the tamer circuit 52 is kept on.
Magnitude of current flowing between the discharge electrode 33 and the wire 31 (hereinafter, called discharge current) is set by a current setter 53 connected to the constant current switch 51. The current setter 53 is structured, for example, with a variable adjustor and the like such as a trimmer and the like. Here, a discharge current value is defined in advance with respect to a resistance value. The constant current switch 51 performs controlling to cause predetermined discharge current to flow based on a resistance value of the current setter 53.
That is, the constant current switch 51 performs controlling so that discharge current of a constant value set by the current setter 53 flows between the discharge electrode 33 and the wire 31 after dielectric breakdown until discharge end.
According to the conventional ball forming device 45 for a wire bonder, a discharge current value of the current setter 53 and a discharge duration of the timer circuit 52 are selected by an operator to be appropriate in accordance with a wire diameter and an initial ball having a predetermined size is to be formed with the selected discharge current value and discharge duration.
In a ball forming device for a wire bonder disclosed in Patent Document 1, for stabilizing in forming an initial ball, a discharge duration from discharge start to discharge end of current flowing between a wire and a discharge electrode is divided into one or more blocks, a discharge current profile for each block is calculated with a specified function from a start current value, an end current value, and a discharge period in each block of the discharge duration, and discharge current is controlled based on the discharge current profiles of the respective blocks calculated at the time of discharge.
Patent Document 2 discloses a ball forming device for a wire bonder in which discharging is stabilized by arranging a fixed resistor for discharge stabilization between a torch electrode and a torch power source as being adjacent to the torch electrode.